Systems and methods for synchronous operation of debris-mitigation devices

ABSTRACT

Systems and methods for synchronous operation of debris-mitigation devices (DMDs) in an EUV radiation source that emits EUV radiation and debris particles are disclosed. The methods include establishing a select relative angular orientation between the first and second DMDs that provides a maximum amount of transmission of EUV radiation between respective first and second rotatable vanes of the first and second DMDs. The methods also include rotating the first and second sets of vanes to capture at least some of the debris particles while substantially maintaining the select relative angular orientation. The systems employ DMD drive units, and an optical-based encoder disc in one of the DMD drive units measures and controls the rotational speed of the rotatable DMD vanes. Systems and methods for optimally aligning the DMDs are also disclosed.

FIELD

The present disclosure relates to debris-mitigation devices such as areused in extreme-ultraviolet-lithography (EUVL) radiation sources, and inparticular relates to systems and methods for the synchronous operationof debris-mitigation devices in EUV radiation sources to optimize thetransmission of EUV radiation.

The entire disclosure of any publication or patent document mentionedherein is incorporated by reference, including the following:US2013/0207,004; U.S. Pat. No. 8,338,797; U.S. Pat. No. 7,302,043; U.S.Pat. No. 7,671,349; and U.S. Pat. No. 6,963,071.

BACKGROUND

Extreme-ultraviolet lithography (EUVL) involves employing anextreme-ultraviolet (EUV) radiation source that generates EUV radiationhaving a wavelength that is typically 13.5 nm+/−2%. The EUV radiation isdirected to a reflective patterned mask to transfer the pattern onto aphotoresist layer supported by a silicon wafer. The use of the smallwavelengths associated with EUV radiation allows for the minimum featuresize of the imaged pattern to also be small, i.e., as small as 15 nm andbelow.

Some EUV radiation sources involve the use of one or more lasers thatdirect respective laser beams to a fuel target to produce a hot plasmathat generates the EUV radiation from an EUV emission location. The EUVradiation is collected by one or more collector mirrors and is thendirected to an intermediate focus.

Unfortunately, the reaction that generates the EUV radiation alsogenerates debris particles (e.g., ions, atoms and clusters of atoms)that can deposit on and into the surfaces of the one or more collectormirrors. The deposited debris particles adversely affect the mirrorreflectance and thus reduce the performance of the EUV radiation source.This contamination of the collector mirrors can happen very rapidly (onthe order of seconds) and can reduce the reflectivity of thecollector-mirror surfaces to the point where the amount of EUV radiationavailable to a downstream illuminator is insufficient to perform the EUVexposure process.

To reduce the adverse effects of the collector contamination from thegenerated debris, it is known in the art to employ a debris-mitigationdevice (hereinafter, DMD). One type of DMD employs rotating vanes thatintercept the debris particles as they travel toward thecollector-mirror surface(s). Because the EUV radiation travels at thespeed of light, the rotating vanes appear stationary for the purposes oftransmitting the EUV radiation, during the transit time of the EUVradiation passing through the DMD. Thus, the reduction in transmissionof the EUV radiation due to the vanes is a function of thecross-sectional area the vanes present to the EUV radiation. Thereduction in the amount of contamination by the debris particles—whichtravel many orders of magnitude slower than the speed of light—is afunction of the speed of the rotating vanes, their axial extent, theenergy (speed) of the debris particles, and the architecture of the DMD,e.g., there may be rotating vanes followed by stationary vanes so thatdebris that does not “stick” to the vanes on the first encounter will bedeflected and have an opportunity to “stick” in a subsequent encounterwith a vane downstream.

SUMMARY

In the industrial application of EUV lithography, it is paramount tomaximize the collection of EUV radiation from the EUV radiation sourcewhile minimizing the degradation of the collector optics by debris fromthe EUV target region. An aspect of the disclosure is coordinating theoperation of two DMDs employed in an EUV radiation source so that theEUV radiation has optimum transmission from the EUV emission locationand through both DMDs to the intermediate focus. One of the DMDs isoperably arranged between the EUV radiation source and anormal-incidence collector (NIC), while the other DMD is operablyarranged between the EUV radiation source and a grazing-incidencecollector (GIC). The two DMDs are aligned and synchronized so that theeffective loss of EUV radiation from the two DMDs is minimized. In oneaspect of the methods disclosed herein, an EUV photon that passes twicethrough the DMD that resides adjacent the NIC will also pass through theDMD adjacent the GIC and then travel through the GIC and be directed tothe intermediate focus IF.

An aspect of the disclosure is a method of operating first and secondDMDs in an EUV radiation source that emits EUV radiation and debrisparticles. The method includes: establishing a select relative angularorientation between the first and second DMDs that provides a maximumamount of transmission of EUV radiation between respective first andsecond rotatable vanes of the first and second DMDs; and rotating thefirst and second sets of vanes to capture at least some of the debrisparticles while substantially maintaining the select relative angularorientation.

Another aspect of the disclosure is a system for performing debrismitigation in an EUV radiation source that emits EUV radiation anddebris particles. The system includes: first and second DMDsrespectively having first and second sets of rotatable vanes andoperably arranged relative to the EUV radiation source so that the EUVradiation passes at least once through each of the first and secondDMDs, wherein the first and second sets of vanes have a select relativeangular orientation that provides a maximum amount of transmission ofEUV radiation through the first and second DMDs; first and second driveunits respectively operably connected to the first and second sets ofvanes; and a controller operably connected to the first and second driveunits and configured to control the first and second drive units tosubstantially maintain the select relative angular orientation duringrotation of the first and second sets of vanes to capture at least someof the debris particles.

Another aspect of the disclosure is an EUV source system that includesthe system as described immediately above and further includes: a fueltarget delivered to an irradiation location; at least one laser thatgenerates a laser beam that irradiates the fuel target to emit the EUVradiation and the debris particles; a GIC arranged adjacent theirradiation location and arranged to receive the EUV radiation anddirect the EUV radiation to an intermediate focus; and an NIC defined bya spherical mirror having a focus at the irradiation location andarranged relative to the irradiation location opposite the GIC mirror sothat the spherical mirror receives and reflects EUV radiation back tothe irradiation location and then to the GIC mirror for redirecting tothe intermediate focus.

Another aspect of the disclosure is an EUV source system for an EUVlithography system that includes along an optical axis: an irradiationlocation to which a Sn target is provided; a GIC having an input endadjacent the irradiation location, an output end, and an intermediatefocus adjacent the output end; a spherical mirror arranged along theoptical axis adjacent the irradiation location opposite the GIC andhaving a focus; at least one laser operably arranged to generate apulsed beam of IR radiation to the irradiation location to irradiate theSn target provided to the irradiation location to form a plasma havingan EUV-emitting region that substantially isotropically emits EUVradiation and that also emits debris particles; a first DMD having afirst set of rotatable vanes and operably arranged between the plasmaand the GIC; a second DMD having a second set of rotatable vanes andoperably arranged between the plasma and the spherical mirror; whereinthe focus of the spherical mirror is located at the EUV-emitting regionof the plasma so that a portion of the emitted EUV radiation passesthrough the second DMD and is received by the spherical mirror and isreflected therefrom back through the second DMD to the EUV-emittingregion and then through the first DMD to the input end of the GIC;wherein the first and second sets of rotatable vanes have a selectalignment that optimizes transmission of the portion of the emitted EUVradiation that travels through the first and second DMDs; and a DMDsynchronization system operably connected to the first and second DMDsand configured to synchronize the rotation of the first and second setsof rotatable vanes to maintain the select alignment of the first andsecond sets of rotatable vanes of the first and second DMDs.

Another aspect of the disclosure is a method of monitoring the operationof a DMD that has a plurality of rotating vanes when the DMD is employedin an EUV source system that generates EUV radiation and debrisparticles. The method includes: monitoring a rotational speed ofrotating vanes during operation of the EUV source system; determining achange in the rotational speed of the rotating vanes due to anaccumulation of debris particles on the rotating vanes; comparing thechange in the rotational speed to a preset change tolerance; andterminating the rotation of the rotating vanes when the change inrotational speed exceeds the preset change tolerance.

Additional features and advantages are set forth in the DetailedDescription that follows and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings. It is to be understood that both theforegoing general description and the following Detailed Description aremerely exemplary and are intended to provide an overview or framework tounderstand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the Detailed Description serve to explain principles andoperation of the various embodiments. As such, the disclosure willbecome more fully understood from the following Detailed Description,taken in conjunction with the accompanying Figures, in which:

FIG. 1A is a schematic diagram of an example EUV source system prior tothe laser beams hitting the fuel target;

FIG. 1B is similar to FIG. 1A but shows the system in the state ofoperation when the laser beams hit the fuel target to create an EUVplasma that generates EUV radiation;

FIG. 2A is a front-on view of an example DMD showing the set of rotatingvanes;

FIG. 2B is a front-on view of an example of the two DMDs of the EUVsource of FIGS. 1A and 1B, illustrating the angular offset of one DMDversus the other with respect to a reference position;

FIG. 2C is a schematic plot of the measured optical power P_(M) versusthe angular offset φ, showing how the angular rotation of one DMDrelative to the other results in peaks in the transmitted power to theIF when the DMDs are properly aligned;

FIG. 3 is a schematic diagram of an example DMD synchronization systemaccording to the disclosure;

FIG. 4 is a close-up elevated view of an example angle encoder accordingto the disclosure;

FIG. 5 is a front-on view of an example encoder disc for the angleencoder of FIG. 4;

FIG. 6 is a close-up and more detailed schematic diagram of the masterdrive unit of the DMD synchronization system of FIG. 3;

FIG. 7 is a schematic diagram of the DMD synchronization system similarto that of FIG. 6 but also including the slave drive unit, and alsoillustrating an example configuration for performing system calibration;and

FIG. 8 is a schematic diagram of an example DMD synchronization systemaccording to the disclosure wherein the servo motors, the angle encodersand the tachometer encoder are integrated with their respective DMDs.

DETAILED DESCRIPTION

Reference is now made in detail to various embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or like reference numbers andsymbols are used throughout the drawings to refer to the same or likeparts. The drawings are not necessarily to scale, and one skilled in theart will recognize where the drawings have been simplified to illustratethe key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute apart of this Detailed Description.

FIGS. 1A and 1B are schematic diagrams of an example EUV radiationsource system (“system”) 10 just prior to the emission of EUV radiation26 and during the emission of the EUV radiation, respectively. Thesystem 10 includes an axis A1 along which is operably arranged agrazing-incidence collector (GIC) 20 that includes an input end 22 andan output end 24. The GIC 20 also includes one or more nestedgrazing-incidence mirrors, and two such mirrors M1 and M2 are shown byway of example.

The system 10 also includes a normal-incidence collector (NIC) mirror 30having a mirror surface 32 that includes a multi-layer reflectivecoating 34. In an example, NIC mirror 30 is spherical and has a focus atan irradiation location IL. The input end 22 of GIC 20 is arrangedrelative to and has a proximal focus at irradiation location IL and alsohas adjacent output end 24 an intermediate focus IF located at or nearan aperture stop AS.

The EUV radiation source system 10 includes at least one laser 50, andtwo such lasers 50A and 50B are shown by way of example. The lasers 50Aand 50B respectively emit laser beams 52A and 52B that are directed to afuel target 23 provided at irradiation location IL by a fuel-targetdelivery system 25. The fuel target 23 may be, for example, a tin (Sn)droplet, and in particular may be a low-mass tin droplet that issubstantially vaporized and ionized when irradiated by laser beams 52Aand 52B.

The system 10 also includes at least one debris-mitigation device (DMD)100. In the example shown in FIGS. 1A and 1B, system 10 includes firstand second DMDs 100, denoted 100A and 100B. The first DMD 100A isoperably disposed between irradiation location IL and input end 22 ofGIC 20. The second DMD 100B is operably disposed between irradiationlocation IL and NIC mirror 30. Example forms of first and second DMDs100A and 100B are discussed in greater detail below. The first andsecond DMDs 100A and 100B are respectively operably connected to DMDdrive units 110 (denoted 110A and 110B), which in turn are operablyconnected to a controller 112.

The controller 112 can be any programmable device used in the art, suchas a computer, micro-controller, FPGA, etc. that can be configured tocontrol the operation of system 10 to perform the methods disclosedherein. In an example, controller 112 includes hardware and softwarethat is configurable to define, in combination with other systemcomponents, one or more control loops, such as phase-lock loops,proportional-integral-derivative loops, and other types offeed-back-based loops. In an example, controller 112 includesinstructions embodied in a computer-readable medium that cause thecontroller to carry out its control functions, including control-loopfunctions, signal processing, etc. In an example, system 10 includesmore than one controller 112.

FIG. 1B shows the state of operation of system 10 after laser beams 52Aand 52B are incident upon fuel target 23 of FIG. 1A. The result of thelaser irradiation of fuel target 23 is the formation of an EUV plasma 29that isotropically emits EUV radiation 26 as well as debris particles 27(e.g., ions and atoms of the fuel material) from an emission region ERlocated substantially at irradiation location IL.

A first portion of EUV radiation 26 emitted by EUV plasma 29 travelsthrough first DMD 100A and is collected by GIC 20 at input end 22 andundergoes a grazing incidence reflection at the GIC surface at leastonce. This grazingly reflected EUV radiation 26 is directed by GIC 20 tointermediate focus IF to form an intermediate image IM. The firstportion of EUV radiation 26 thus defines a first optical path OP1 fromemission region ER to intermediate focus IF.

Another portion of EUV radiation 26 emitted by EUV plasma 29 isenvisioned to initially travel in the opposite direction of the firstportion and through second DMD 100B to NIC mirror 30 over a secondoptical path OP2. This EUV radiation 26 reflects from NIC surface 32 andtravels back through second DMD 100B over substantially the same secondoptical path OP2 back to emission region ER—because the shape of NICmirror 30 is a sphere with its center at the emission region—so thatradiation from the emission region is reflected by the NIC surface backonto itself. This EUV radiation 26 then continues along first opticalpath OP1 as if, like the first portion of EUV radiation, it wereinitially emitted from emission region ER.

Thus, a portion of second optical path OP2 overlaps first optical pathOP1 so that the second portion of EUV radiation 26 also travels throughfirst DMD 100A to GIC 20 and then to intermediate focus IF, therebycontributing to the formation of intermediate image IM. Since EUVradiation 26 travels at the speed of light, the vanes of the two DMDs100A, 100B are essentially stationary during the passage of the EUVradiation through the DMD regions. Thus it is of particular importancefor the optimization of the debris-mitigation process that the first andsecond optical paths OP1 and OP2 overlap when traveling through firstDMD 100A, as described below.

FIG. 2A is a front-on view of an example DMD 100. The DMD 100 includes ahousing 102 that operably supports a set of vanes 104 that are rotatablewithin the housing about a central axis. The vanes 104 define aplurality of apertures 106 through which the EUV radiation 26 can pass.An example DMD 100 is disclosed in U.S. Pub. No. 2012/0305810. Therotation of vanes 104 in DMD 100 serves to prevent the surfaces ofmirrors M1 and M2 of GIC 20 from being coated by debris particles 27.The vanes 104 intercept debris particles 27. The rotating vanes 104 thusserve to sweep out a substantial portion of debris particles 27 beforethey reach GIC 20 and NIC mirror 30.

The material of vanes 104 can block EUV radiation 26 and so represents asource of attenuation for the EUV. The rotating vanes 104 are thuspreferably thin (in the transverse direction, i.e., in the θ rotationaldirection) to minimize the amount of EUV radiation 26 that is blocked bythe vane edges (e.g., thin enough so that they block no more than 20% ofthe EUV radiation) and longer in the axial direction to maximize theinterception and capture of the slower moving debris particles 27.

The first and second DMDs 100A and 100B define respective attenuationsAT_(A) and AT_(B) of EUV radiation 26 due to their respectivecross-sectional areas defined by vanes 104. In the case where DMDs 100Aand 100B have identical vane configurations (at least with respect tothickness and number), then AT_(A)=AT_(B). For ease of discussion, it isassumed that vanes 104 of DMDs 100A and 100B are similar to the pointwhere AT_(A)=AT_(B)=AT.

If vanes 104 in the two DMDs 100A and 100B are identical and aligned(e.g., both open at the same time), then the second portion of EUVradiation 26 that travels over the double-pass optical path OP2 throughsecond DMD 100B experiences only a single attenuation from the vanestherein when passing through the DMDs. This is because EUV radiation 26travels at the speed of light and thus makes the round-trip over secondoptical path OP2 in a time so short that vanes 104 have no appreciablemovement. Any blockage of the second portion of EUV radiation 26 byvanes 104 in second DMD 100B occurs only on the first passage of thesecond portion of the EUV radiation through the second DMD.

An example DMD 100 can have about 180 vanes 104 that are each 0.1 mmwide in the azimuthal direction. This DMD 100 can be used to reduce thenumber of fast debris particles 27 (e.g., particles traveling at about2.5×10⁵ cm/s) with vanes 104 that are 25 cm long in the axial directionand have rotational speeds of about 3,300 RPM. (In another examplerotating vanes 104 that are 10 cm long will require a rotationalfrequency of about 8,300 RPM to achieve the same debris-mitigationperformance.) This configuration for DMD 100 blocks about 15% of EUVradiation 26 just from its static shadow; and it will sweep out alldebris particles 27 moving slower than about 2.5×10⁵ cm/s. In anexample, some vanes 104 can be made stationary and positioned downstreamof other rotating vanes to enhance the collection of deflected debrisparticles 27.

DMD Synchronization

The first and second DMDs 100A and 100B can operate within system 10without synchronization. In such operation, however, the unsynchronizedrotation of vanes 104 of the two DMDs 100A and 100B will result in anoverall increase in the EUV radiation attenuation compared to that ofaligned and synchronized DMDs. That is because in the un-aligned andun-synchronized case EUV radiation 26 that is headed toward NIC mirror30 will undergo the attenuation due to passage through DMD 100B and willthen undergo an additional attenuation when passing through DMD 100Aafter its reflection from NIC mirror surface 32.

When the two DMDs 100A and 100B are aligned and synchronized for maximumtransmission, EUV radiation 26 that is headed toward NIC mirror 30 willundergo the attenuation due to passage through DMD 100B but will undergo(none to minimal) additional attenuation when passing through DMD 100Aafter its reflection from NIC mirror surface 32. So, proper alignmentand then synchronization (with feedback control to maintain thesynchronicity) of the rotation of vanes 104 in first and second DMDs100A and 100B can be used to limit the total DMD attenuation. Dependingon the specific values of DMD alignment and transmissions, and on mirrorcollection solid angles and reflectivities, the aligned and synchronizedcondition can result (for typical system parameters) in an increased EUVpower at intermediate focus IF of about 10% over the randomly un-alignedand un-synchronized case.

FIG. 2B is a front-on view of first and second DMDs 100A and 100B. Thefirst DMD 100A is shown as being rotationally aligned to a referenceposition REF. The second DMD 100B is shown as having a rotation angle φwith respect to first DMD 100A. The example first and second DMDs areeach shown as having 16 vanes 104 for ease of illustration. The 16 vanes104 define 16 apertures 106. The 16 vanes 104 have angular separationsof 2π/16=π/8 radians. Thus, if first and second DMDs 100A and 100B startout rotationally aligned (φ=0) and then one DMD is rotated by a rotationangle of φ=π/8, the two DMDs return to being rotationally aligned. For Nvanes 104, the rotation angle φ that returns the two DMDs 100A and 100Bto alignment is 2π/N.

In the case where one DMD has a different number of vanes 104, therotation angle that returns the two DMDs 100A and 100B to alignment canbe determined readily by knowing the particular configuration of eachDMD. In either case, there will be a limited range of the rotation anglethrough with the two DMDs 100A and 100B can be rotated to achieve aselect relative alignment.

FIG. 2C is a schematic plot of the measured optical power atintermediate focus IF versus the relative rotation angle φ between firstand second DMDs 100A and 100B. The plot shows that there is a relativerotation angle φ, denoted φ_(MAX), where the measured optical power isat a maximum. This occurs when vanes 104 of first and second DMDs 100Aand 100B are aligned in a manner that provides minimum blockage of EUVradiation 26 over first and second optical paths OP1 and OP2. This anglefor maximum transmitted optical power is not necessarily φ=0. When firstand second DMDs 100A and 100B are identical, then φ_(max)=2π/N, where Nis the number of vanes 104 in each of the two DMDs. How the selectrelative angular alignment of vanes 104 of first and second DMDs 100Aand 100B is determined is discussed in greater detail below.

FIG. 3 is a schematic diagram of an example DMD synchronization system200 according to the disclosure. The DMD synchronization system 200includes first and second DMDs 100A and 1006, the respective drive units110A and 1106, and controller 112. The drive unit 110A includes a servomotor 210A operably connected to a drive shaft 212A to which is operablyattached an angle encoder 214A. The drive shaft 212A leads to a transferbox 216A configured to drive a drive shaft 220A to drive the rotation ofvanes 104 in DMD 100A about axis A1. The drive unit 110A also includes adrive amplifier 230A operably connected to servo motor 210A.

The drive unit 1106 is configured essentially the same as drive unit110A and includes a servo motor 2106, a drive shaft 212B, an angleencoder 214B, a transfer box 216B, and a drive shaft 220B. The driveunit 1106 also includes a tachometer encoder 222B operably connected todrive shaft 2126. The drive unit 1106 also includes a drive amplifieroperably connected to servo motor 210B and to tachometer encoder 222B.The controller 112 of DMD synchronization system 200 includes or isotherwise configured as a phase-lock loop (PLL) that is electricallyconnected to the two angle encoders 214A and 214B and to drive amplifier230A of drive unit 110A.

The configuration of DMD synchronization system 200 makes drive unit1106 the primary or master drive unit and drive unit 110A the secondaryor slave drive unit. The master drive unit 1106 operates as aconstant-velocity tachometer loop, while secondary drive unit 110Aoperates as a synchronous phase loop.

The DMD synchronization system 200 is configured to maintain therelative angular locations of vanes 104 during the operation of firstand second DMDs 100A and 100B so that the attenuation of EUV radiation26 due to the vanes in both DMDs is minimized. This requires firstdetermining the angular locations (relative to reference angularlocation REF) of vanes 104 of each of first and second DMDs 100A and100B so that EUV radiation 26 that double-passes through the second DMDalso passes through the first DMD. This can be accomplished by using aray trace calculation of the kind that is known to one skilled in theart and that is available on most commercially available lens-designsoftware programs. This can also be accomplished empirically, asdiscussed in greater detail below in connection with FIG. 8. The selectalignment needs to be determined only once for the given configurationof system 10.

Once the optimum alignment of first and second DMDs 100A and 100B isestablished, the relative angular orientation of the DMDs needs to betracked during the rotation of vanes 104 in each of the first and secondDMDs. In the configuration of DMD synchronization system 200 of FIG. 3,this is accomplished by angle encoders 214A and 214B measuring therespective angular positions (i.e., first and second angular positions)of respective drive shafts 212A and 212B. The angular positioninformation is then used to determine the respective rotational speedsof drive shafts 212A and 212B (e.g., using controller 112).

If the measured angular orientation shifts beyond a given angulartolerance, this gives rise to a phase error Δφ. This phase error Δφ isprovided to the PLL of controller 112. In response, controller 112 sendsa control signal to drive amplifier 230A to change the speed of servomotor 210A to reduce the phase error Δφ (i.e., to drive Δφ to zero) tobring the relative angular orientations of first and second DMDs 100Aand 100B back within the angular tolerance.

Example Angle Encoder

FIG. 4 is a close-up elevated view of an example angle encoder 214. Theangle encoder 214 includes an encoder disc 250 that has a perimeter 251and front and back surfaces 252 and 254. The encoder disc 250 isconfigured to rotate with drive shaft 212. A capture plate 260 isoperably arranged relative to front surface 252 and perimeter 251 ofencoder disc 250 and defines a number of apertures 262 relative toradial positions on the encoder disc. A plurality of light sources 280and a corresponding plurality of photodetectors 282 are respectivelyradially arranged adjacent front and back surfaces 252 and 254 ofencoder disc 250 so that light source and photodetector pairs are inoptical communication through respective apertures 262 and through theencoder disc.

FIG. 5 is a front-on view of an example encoder disc 250. The encoderdisc 250 includes annular sections 253 that each includes a periodicarray of transmissive regions 256 and opaque regions 258. Theperiodicity of transmissive regions 256 and opaque regions 258 ofannular sections 253 increases with the radius of the encoder disc 250.The example encoder disc 250 includes eight annular regions 253, withthe size of transmissive regions 256 and opaque regions 258 decreasingby a factor of two for each annular region moving from the centeroutward. Thus, annular sections 253 are binary encoded with binaryoptical transmission patterns that increase in periodicity with theradius.

The annular regions 253 are radially aligned with light sources 280 andcorresponding photodetectors 282 so that light from a given light sourcehas to pass through an intervening annular region. Because encoder disc250 spins relative to the stationary light sources 280 and theircorresponding detectors 282, transmissive regions 256 and opaque regions258 give rise to a modulation that is radially dependent. This allowsfor dynamic coarse-to-fine angular measurement resolutions and theabsolute tracking of DMDs 100 and vanes 104 therein.

Drive Shaft Synchronization

An aspect of the disclosure involves providing rotationalsynchronization between drive shafts 212A and 212B, which aremechanically isolated and motor driven by separate servo motors 210A and210B. As noted above, the primary or master drive unit 110B isconfigured in a speed control loop using tachometer encoder 222B. Thetachometer encoder 222B is sensitive only to rotational speed. A driveamplifier 230B can be configured in a standard proportional control loopwith tachometer encoder 222B providing feedback. As an alternative, aproportion integral-derivative (PID) loop configuration can beimplemented.

FIG. 6 is a close-up and more detailed schematic diagram of master driveunit 110B of DMD synchronization system 200 of FIG. 3 and shows moredetails about how the master drive unit maintains the rotation rate ofDMD vanes 104 at a constant speed in a control loop configuration. Inthe example shown, tachometer encoder 222B measures the rotational speedS_(M) of drive shaft 212B. Here, it is assumed that this rotationalspeed is the same as or is directly proportional to the rotational speedof vanes 104 in DMD 100B (e.g., via the operation of transfer box 216B).The measured rotational speed S_(M) is provided as one input to acomparator 300. A speed control unit 310 is used to provide an inputrotational speed S_(I). This input rotational speed S_(I) is provided toa set-point unit 316 that provides an input voltage to the other inputof comparator 300. The comparator 300 provides an output error voltagesignal V_(E) representative of the difference between the measuredrotational speed S_(M) and the input (desired) rotational speed S_(I).The error voltage V_(E) and input speed S_(I) are combined at 320 andprovided to drive amplifier 230B, which changes the speed of servo motor210B to correct for the measured error in the rotational speed tomaintain the rotational speed at the input speed S_(I).

Thus, the configuration of master drive unit 1106 defines a control loopthat maintains servo motor 210B at a set rotational speed even undervarying load conditions, such as when vanes 104 accumulate debrisparticles 27 and become heavier. As an alternative, a PID loop can beused.

Single DMD Monitoring

It is noted that an aspect of the disclosure is directed to monitoringthe angular rotation and phase of a single DMD 100 using angle encoder214. This monitoring can be useful because rotating vanes 104 can becomeloaded with debris 27 (e.g., condensed Sn). This could load servo motor210 and lead to rotational instability of the rapidly rotating vanes104. Thus, the methods described herein including monitoring andmaintaining the speed of a single (master) DMD 100 in the embodimentwhere only a single DMD is used. Such an embodiment may be for anexample system 10 where GIC 20 is the only collector (i.e., no NICmirror 30 is used).

In an example, the error voltage signal V_(E) is monitored for thesingle DMD 100 during the operation of system 10. When the error voltageV_(E) exceeds a preset value or tolerance (e.g., due to debrisaccumulation on vanes 104 of the single DMD 100), a warning message(e.g., “maintenance required,” or “service” or the like) can begenerated by controller 112 and the DMD serviced. In an example, whenthe change in the rotational speed of vanes 104 exceeds the presentchange tolerance, the operation of DMD 100 is terminated (i.e., therotation of the vanes is terminated) to avoid damaging the DMD and/orDMD drive unit 110.

DMD Synchronization with Phase-Lock Loop

FIG. 7 is similar to FIG. 6 and adds secondary or slave drive unit 110A.Synchronizing master drive unit 1106 and slave drive unit 110A can beaccomplished by creating an angular position feedback system usingbinary encoders or analog resolvers. As illustrated in FIG. 7,controller 112 has a phase-lock loop (PLL) that provides the measuredangular phase error Δφ to slave drive unit 110A to synchronize theangular position with that of master drive unit 1106. Thus, servo motor230B of master drive unit 1106 is driven at a constant speed and thecontrol loop is used to adjust the speed of servo motor 230A of slavedrive unit 110A as needed to maintain the angular phase error Δφ withina select tolerance, and in an example at Δφ=0.

An example aspect of using DMD synchronization system 200 can includeperforming an alignment and calibration procedure assembly to alignangle encoders 214 and to set the loop coefficients to match loading.The drive and loop circuitry can be designed such that most calibrationsand alignments can be done electronically by means of offset and gainparameters. This can enhance computer control capabilities as well.

In the example illustrated in FIG. 7, an additional optical or radiantenergy feedback control loop may be implemented to perform systemcalibration and help stabilize the exposure energy and also to informsystem 200 when vanes 104 are attenuating too much of EUV radiation 26energy due to debris build up or are otherwise creating attenuationbeyond an expected value. In an example, a detector 350 is operablyarranged within system 200 so it that can measure the optical powerP_(M) at a convenient position downstream of first DMD 100A. The signalfrom detector 350 representative of the measured power P_(M) can be fedto controller 112 and then processed to provide updated speed controlinformation SU to speed control unit 310. This feedback mechanism can beused to assist in optimizing the transmission of EUV radiation 26through DMDs 100A and 100B and to intermediate focus IF.

Establishing a Select DMD Alignment

As discussed above, a select alignment of first and second DMDs 100A and100B that provides optimum transmission of EUV radiation 26 through theDMDs needs to be established prior to operating system 10. In anexample, this can be performed by measuring the optical power P_(M) withdetector 350 while adjusting the relative rotation angle φ of first andsecond DMDs 100A and 100B. The rotation angle φ that provides a maximummeasured power P_(M) can be used as the angle that provides the selectalignment.

It is also noted that the above method for determining a selectalignment can be done with a radiation source other than EUV plasma 29.Because system 10 is a mirror-based system, it has no chromaticaberration. Consequently, an alternate light source 324 that emits light326 of a different wavelength (e.g., such as a visible wavelength) or abroad range of wavelengths can be used in place of EUV plasma 29 todetermine the select alignment. The alternative light source 324 shouldbe one that, like EUV plasma 29, allows for light traveling over opticalpath OP2 to pass through it. Thus, light source 324 can be incandescent,a flame, another type of plasma, fluorescent, one or more LEDs, etc.

Integrated DMD Synchronization System

FIG. 8 is a schematic diagram that illustrates an alternate embodimentof DMD synchronization system 200 that shows a configuration where servomotors 210A and 210B are integrated with their corresponding DMDs 100Aand 100B in a manner that eliminates the need for drive shafts 212 and220. The tachometer encoder 222B and angle encoders 214A and 214B arealso integrated into their respective servo motors.

It will be apparent to those skilled in the art that variousmodifications to the preferred embodiments of the disclosure asdescribed herein can be made without departing from the spirit or scopeof the disclosure as defined in the appended claims. Thus, thedisclosure covers the modifications and variations provided they comewithin the scope of the appended claims and the equivalents thereto.

What is claimed is:
 1. A method of operating first and seconddebris-mitigation devices (DMDs) in an extreme-ultraviolet (EUV)radiation source that emits EUV radiation and debris particles,comprising: establishing a select relative angular orientation betweenthe first and second DMDs that provides a maximum amount of transmissionof EUV radiation between respective first and second rotatable vanes ofthe first and second DMDs; and rotating the first and second sets ofvanes to capture at least some of the debris particles whilesubstantially maintaining the select relative angular orientation. 2.The method according to claim 1, wherein a variation from the selectrelative angular orientation defines a phase error, and whereinmaintaining the select relative angular orientation is based on ameasurement of the phase error.
 3. The method according to claim 2,further including: rotating the first set of vanes at a first speed thatis substantially constant; and rotating the second set of vanes at asecond speed that is adjustable to reduce the phase error.
 4. The methodaccording to claim 3, further comprising using the phase error in acontrol loop when adjusting the second speed of the second set of vanes.5. The method according to claim 1, including determining first andsecond rotational speeds of the first and second sets of vanes usingfirst and second angular position information from first and secondangle encoders.
 6. The method according to claim 5, wherein the firstand second angle encoders comprise respective first and second binaryencoders, with each of the first and second binary encoders having anencoder disc with annular binary optical transmission patterns withperiods that increase with the radius of the encoder disc.
 7. The methodaccording to claim 1, wherein the rotating of the first and second setsof vanes is accomplished by respective first and second servo motorsthat include respective first and second drive shafts that are operablyconnected to the first and second sets of vanes.
 8. A system forperforming debris mitigation in an extreme-ultraviolet (EUV) radiationsource that emits EUV radiation and debris particles, comprising: firstand second debris-mitigation devices (DMDs) respectively having firstand second sets of rotatable vanes and operably arranged relative to theEUV radiation source so that the EUV radiation passes at least oncethrough each of the first and second DMDs, wherein the first and secondsets of vanes have a select relative angular orientation that provides amaximum amount of transmission of EUV radiation through the first andsecond DMDs; first and second drive units respectively operablyconnected to the first and second sets of vanes; and a controlleroperably connected to the first and second drive units and configured tocontrol the first and second drive units to substantially maintain theselect relative angular orientation during rotation of the first andsecond sets of vanes to capture at least some of the debris particles.9. The system according to claim 8, wherein the first and second driveunits include respective first and second angle encoders that measure aphase error between the select relative angular orientation and ameasured relative angular orientation, and wherein the controllerutilizes the phase error in a control loop in said maintaining of theselect relative angular orientation.
 10. The system according to claim9, wherein the first drive unit is configured as a master unit thatrotates the first set of vanes at a first speed that is substantiallyconstant, and wherein the second drive unit is configured as a slaveunit that rotates the second set of vanes at a second speed that variesbased on the measured phase error.
 11. The system according to claim 9,wherein the first and second angle encoders are each operably attachedto respective first and second drive shafts and wherein each of thefirst and second angle encoders includes: a) an encoder disc withannular binary optical transmission patterns having periods thatincrease with the radius of the encoder disc, and b) light sources anddetectors operably arranged relative to the encoder disc to measurerespective modulations of the binary optical transmission patterns dueto the rotation of the encoder disc.
 12. The system according to claim9, wherein the first drive unit includes: a servo motor thatrotationally drives the first set of vanes; a drive amplifier operablyconnected to the servo motor; a tachometer encoder that measures arotational speed of the first set of vanes; a comparator operablyconnected to the tachometer encoder and that compares an inputrotational speed to the measured rotational speed to define an errorvoltage representative of the difference between the measured rotationalspeed and the input rotational speed; and wherein the drive amplifierreceives the error voltage and causes the servo motor to change therotational speed of the first set of vanes to reduce the error voltage.13. The system according to claim 9, further including a speed controlunit operably connected to the comparator and that provides the inputrotational speed.
 14. An EUV source system, comprising: the system ofclaim 8; a fuel target delivered to an irradiation location; at leastone laser that generates a laser beam that irradiates the fuel target toemit the EUV radiation and the debris particles; a grazing incidencecollector (GIC) arranged adjacent the irradiation location and arrangedto receive the EUV radiation and direct the EUV radiation to anintermediate focus; and a normal incidence collector (NIC) defined by aspherical mirror having a focus at the irradiation location and arrangedrelative to the irradiation location opposite the GIC mirror so that thespherical mirror receives and reflects EUV radiation back to theirradiation location and then to the GIC mirror for redirecting to theintermediate focus.
 15. An extreme-ultraviolet (EUV) source system foran EUV lithography system, comprising along an optical axis: anirradiation location to which a Sn target is provided; agrazing-incidence collector (GIC) having an input end adjacent theirradiation location, an output end, and an intermediate focus adjacentthe output end; a spherical mirror arranged along the optical axisadjacent the irradiation location opposite the GIC and having a focus;at least one laser operably arranged to generate a pulsed beam ofinfrared (IR) radiation to the irradiation location to irradiate the Sntarget provided to the irradiation location to form a plasma having anEUV-emitting region that substantially isotropically emits EUV radiationand that also emits debris particles; a first debris-mitigation device(DMD) having a first set of rotatable vanes and operably arrangedbetween the plasma and the GIC; a second DMD having a second set ofrotatable vanes and operably arranged between the plasma and thespherical mirror; wherein the focus of the spherical mirror is locatedat the EUV-emitting region of the plasma so that a portion of theemitted EUV radiation passes through the second DMD, is received by thespherical mirror and is reflected therefrom back through the second DMDto the EUV-emitting region and then through the first DMD to the inputend of the GIC; wherein the first and second sets of rotatable vaneshave a select alignment that optimizes transmission of the portion ofthe emitted EUV radiation that travels through the first and secondDMDs; and a DMD synchronization system operably connected to the firstand second DMDs and configured to synchronize the rotation of the firstand second sets of rotatable vanes to maintain the select alignment ofthe first and second rotatable vanes of the first and second DMDs. 16.The system of claim 15, wherein the first and second DMDs have identicalvane configurations.
 17. The system of claim 15, wherein the DMDsynchronization system includes: first and second drive unitsrespectively operably connected to the first and second sets ofrotatable vanes; and a controller operably connected to the first andsecond drive units and configured to control the first and second driveunits to substantially maintain the select alignment.
 18. The systemaccording to claim 17, wherein the first and second drive units includerespective first and second angle encoders that measure a phase errorbetween the select alignment and a measured alignment, and wherein thecontroller utilizes the phase error in a control loop in saidmaintaining of the select alignment.
 19. The system according to claim18, wherein the first drive unit is configured as a master unit thatrotates the first set of rotatable vanes at a first speed that issubstantially constant, and wherein the second drive unit is configuredas a slave unit that rotates the second set of rotatable vanes at asecond speed that varies based on the measured phase error.
 20. Thesystem according to claim 19, wherein the first and second angleencoders are each operably attached to respective first and second driveshafts and wherein each of the first and second angle encoders includes:a) an encoder disc with annular binary optical transmission patternshaving periods that increase with the radius of the encoder disc, and b)light sources and detectors operably arranged relative to the encoderdisc to measure respective modulations of the binary opticaltransmission patterns due to the rotation of the encoder disc.
 21. Thesystem according to claim 20, wherein the first drive unit includes: aservo motor that rotationally drives the first set of vanes; a driveamplifier operably connected to the servo motor; a tachometer encoderthat measures a rotational speed of the first set of rotatable vanes; acomparator operably connected to the tachometer encoder and thatcompares an input rotational speed to the measured rotational speed todefine an error voltage representative of the difference between themeasured rotational speed and the input rotational speed; and whereinthe drive amplifier receives the error voltage and causes the servomotor to change the rotational speed of the first set of rotatable vanesto reduce the error voltage.
 22. The system according to claim 21,further including a speed control unit operably connected to thecomparator and that provides the input rotational speed.
 23. A method ofmonitoring the operation of a debris-mitigation device (DMD) that has aplurality of rotating vanes when employed in an extreme-ultraviolet(EUV) source system that generates EUV radiation and debris particles,comprising: monitoring a rotational speed of rotating vanes duringoperation of the EUV source system; determining a change in therotational speed of the rotating vanes due to an accumulation of debrisparticles on the rotating vanes; comparing the change in the rotationalspeed to a preset change tolerance; and terminating the rotation of therotating vanes when the change in rotational speed exceeds the presetchange tolerance.
 24. The method of claim 23, wherein the change in therotational speed of the rotating vanes is measured as a voltage, andwherein the preset change tolerance is provided as an error voltage. 25.The method of claim 23, further including generating a warning messagewhen the change in rotational speed exceeds the preset change tolerance.